1. Field of the Invention
The present invention relates to an artificial tooth root selectively imparted with acid resistance and with anti-adherence of sordes and various germs in areas where a periodontal pocket is likely to occur, and to a production process therefor. More particularly, the invention relates to a novel surface treatment technique for selective surface treatment of a predetermined area of a dentistry material, which can give a properties of anti-adherence of sordes such as tobacco tar and various germs to a predetermined area of the surface of an artificial tooth root coated with calcium phosphate based ceramic having bioaffinity.
2. Description of the Related Art
Periodontal disease and similar disorders stem from infections in the space of tooth and gingival, i.e., periodontal pockets. In many cases, infection recurs due to infiltration of various germs from the area around a repaired tooth or dental crown. Kasuga et al. succeeded in imparting antibacterial properties to the surface of crystalline glass for artificial crowns by introducing silver ions by means of ion exchange (Material Integration (1999), Vol. 12, No. 5, p. 27). The resultant surface exhibits high antibacterial properties against Escherichia coli and Staphylcoccus aureus. 
Yoshino et al, with the aim of preventing adherence of sordes to surfaces of dentistry materials such as dental prostheses, employed a silane coupling agent with a fluorocarbon chain in order to modify the surface of a substrate (Kagaku to Kogyc(1993), Vol. 46, No. 12, p, 58). Depending on the nature of the substrate treated, the resultant surface exhibits water repellence as high as 100 to 115xc2x0, reducing adherence of sordes.
Through these methods, it has been possible to control adherence of various germs or contaminant of the dentistry materials. However, the former process, while imparting exceptional antibacterial properties, has no effect whatsoever in terms of inhibiting adherence of sordes. Surfaces treated by the latter process exhibit a water droplet contact angle of around 100 to 115xc2x0. However, with a contact angle on this order, there is 30% contact between the surface and a water droplet, so with extended use adherence of sordes occurs, and there is a deterioration of initial performance. Further, the process is decidedly not xe2x80x9cenvironmentally friendlyxe2x80x9d, requiring a large number of processing steps, using copious amounts of water, using organic solvents, etc. Neither of the processes described above is readily adapted to selective surface treatment of a predetermined area.
Calcium phosphate based-ceramics have a number of advantages, such as earlier ossification than do titanium implants, and direct bonding with bone. Clinical application of these materials as dental implants is advancing. However, these ceramics have low strength and tend to be brittle, making use per se as implants difficult. Accordingly, calcium phosphate based ceramics are typically coated onto titanium or a titanium alloy by a plasma spraying process or the like for use. However, differences in the coefficient of thermal expansion between titanium or titanium alloys and the coating film give rise to residual stress, resulting in delamination from the substrate, loss of the film through dissolution of the coating film due to depressed pH in the adjacent area, and other problems; thus, over the long term, titanium implants offer greater reliability. It has been reported by Katayama et al. that depressed pH tends to occur within periodontal pockets, with pH dropping on average to around 6 (Hotei Shi (1986), Vol. 30, p. 665).
With the foregoing in view, thoroughgoing research conducted by the inventors has led to the discovery that introduction of fluorocarbon chains or long-chain alkyl groups to a predetermined area of the surface of a calcium phosphate based ceramic coating produced on a titanium or titanium alloy implant by means of plasma spraying etc., that is, to an area susceptible to formation of a periodontal pocket, appreciably improves water repellence, appreciably reduces adherence of sordes and various germs, and appreciably improves resistance to acid. The present invention was perfected on the basis of this discovery.
Specifically, the present invention provides an artificial tooth root particularly effective as a dentistry material imparted in a predetermined area of the implant with exceptional resistance to adherence of sordes and various germs, as well as with resistance to acid, and offering appreciably reduced dissolution of the calcium phosphate based ceramic in portions thereof exposed within a periodontal pocket. Further provided is a process for the production thereof.
The present invention provides an artificial tooth root having acid resistance and anti-adherence of sordes and various germs, as well as a process for the production thereof.
The invention relates to an artificial tooth root which is a dentistry material consisting of a substrate and a coating of calcium phosphate based ceramic having bioaffinity formed on the surface of this substrate, the substrate is composed of a polymer, ceramic, metal, or other material, wherein the surface of the ceramic coating on the substrate surface is endowed with a protrusion-and-recession configuration and is chemically modified, and to a production process for the artificial tooth root endowed with acid resistance and with anti-adherence of sordes and various germs, comprising the steps of forming a coating of calcium phosphate based ceramic having bioaffinity over a substrate while imparting protrusions and recessions thereto, masking an area of the ceramic coating surface of which bioaffinity is required by a material, and then fixing a silane coupling agent exclusively over a predetermined area of the ceramic coating surface, namely, an area where a periodontal pocket is likely to occur.
To solve the problems described above, the present invention is composed of the following technical means.
(1) An artificial tooth root with acid resistance and anti-adherence of sordes and various germs, which comprises of a substrate and a coating of calcium phosphate based ceramic having bioaffinity formed on the surface of the substrate,
the substrate is composed of a polymer, ceramic, metal, or other material,
the surface of the ceramic coating on the substrate surface is endowed with a protrusion-and-recession configuration, and is chemically modified.
(2) The artificial tooth root according to (1) above, wherein the protrusions and recessions provided to the ceramic coating surface have a ridge-to-trough distance of 50 nm or greater.
(3) The artificial tooth root according to (1) above, wherein the silane coupling agent is fixed over a predetermined area of the ceramic coating surface situated in proximity to the boundary of the gum and the dental crown, where is susceptible to bacterial infiltration and to gingivitis.
(4) The artificial tooth root according to (1) above, wherein the silane coupling agent contains a fluorocarbon chain, long-chain alkyl group, or other hydrophobic group.
(5) A method for producing the artificial tooth root with acid resistance and anti-adherence of sordes and various germs as defined in (1) above, comprising the steps of
forming a coating of calcium phosphate based ceramic having bioaffinity over a substrate while imparting protrusions and recessions thereto,
masking an area of the ceramic coating surface of which bioaffinity is required by a material,
and then fixing a silane coupling agent exclusively over a predetermined area of the ceramic coating surface, where a periodontal pocket is likely to occur.
The invention is now described in greater detail.
The most notable feature of the invention resides in the element of producing a coating of calcium phosphate based ceramic having bioaffinity layer over a titanium or titanium alloy substrate by means of a dipping process, electrophoresis, flame spraying process, plasma spraying process, or the like, and fixing by means of a chemical reaction (namely, dehydration/condensation reaction) a silane coupling agent having a hydrophobic group, such as a fluorocarbon chain, long-chain alkyl group, etc., to a predetermined area of the coating layer, namely, an area where a periodontal pocket is likely to occur.
Substrates useable in the present invention include those arbitrarily selected for use from among titanium, titanium alloys, ceramics, polymers, stainless steel, etc. The use of titanium alloys is preferred. As specific examples of substrates, a hollow circular artificial tooth root or a screw type artificial tooth root, for example, may be given by way of example. Examples of calcium phosphate based ceramics having bioaffinity that may be used include inter alia apatite hydroxide, xcex2-tricalcium phosphate (xcex2-TCP), xcex1-TCP, tetracalcium phosphate (TeCP), etc. Formation of the coating layer may be accomplished by any of a number of processes such as dipping, electrophoresis, flame spraying, plasma spraying, etc. In preferred practice, a plasma spraying process is preferred. As regards materials that may be used with plasma Spraying processes, calcium phosphate based ceramic powders may be used. To enhance fixation of the coating layer to the substrate, a mixture of pure titanium powder and calcium phosphate based ceramic powder in any desired ratio may be used. The particle size of tie powder used is not critical, but in preferred practice will be from 0.1 to 300 microns. In preferred practice, to ensure the presence of an air layer in spaces between the protrusion-recession, the protrusions and recessions provided to the surface of the coating layer will have a ridge-to-trough distance of 50 nm or greater. For plasma spraying, an atmospheric pressure plasma spraying process, partial vacuum plasma spraying process, or the like may be used. In preferred practice, an atmospheric pressure plasma spraying process, which permits high temperatures to be produced, will be used. Types of plasma include high frequency plasma, DC plasma, microwave plasma, etc. In preferred practice, high frequency plasma is preferred as there is not risk of contamination of the coating by wear of the electrode material, etc.
Next, the substrate surface on which a layer of calcium phosphate based ceramic having bioaffinity has been formed as noted is cleaned with ultraviolet radiation, ozone, plasma, etc., to remove any organic matter adhering to the surface. Here, the use of short-wavelength vacuum ultraviolet (a 172 nm excimer lamp) is preferred. Next, the dehydration/condensation reaction among hydroxyl groups present on the coating layer surface and hydroxyl groups present in the silane coupling agent having a hydrophobic group, e.g., fluorocarbon chain or long-chat alkyl group, is utilized to bring about secure fixation of these silane coupling agents to the coating layer surface. The fixing method is not critical, but in preferred practice will be a chemical vapor phase surface modification process, which does not require costly reaction equipment, extended treatment times, or the use of organic solvents, and which affords treatment with extremely small amounts of material. Here, in preferred practice, treatment temperature is from 100 to 150xc2x0 C. and treatment time from 1 to 3 hours.
Bioaffinity is required in areas other than a predetermined area, namely, the area susceptible to formation of a periodontal pocket, so these areas are masked prior to fixing the silane coupling agent. Masking may be accomplished, for example, by applying a polyimide based resin and curing for 2 to 60 minutes at 350 to 400xc2x0 C. The masking agent is removed with a phenolic stripping agent once the silane coupling agent has become fixed. The area to be masked will differ by individual depending on bone morphology, etc., but in preferred practice will extend 12 mm from the tip of the artificial tooth root. By so doing, the area that will contact the bone (the masked area) is imparted with bioaffinity, thereby promoting ossification, while the area susceptible to formation of a periodontal pocket (the non-masked area is imparted with appreciably acid resistance and anti-adherence of sordes and various germs.
In this way, the invention described herein provides selective surface treatment of a predetermined area of an artificial tooth root by means of a simple process.
The reason that predetermined area of the treated substrate exhibits high resistance to adherence of sordes and various germs and resistance to acid is thought to lie in a synergistic effect on the part of chemical effects produced by the fluorocarbon chains or long-chain alkyl groups fixed on the coating layer surface, and physical effects resulting from the presence of protrusions and recessions on the coating layer surface.